1. Field of the Invention
The invention relates to a grinding wheel comprising grains, a synthetic resin-based, preferably duroplastic binding agent and preferably filler material with a high-strength fibre reinforcement which is embedded in the grinding body.
2. Description of the Prior Art
In grinding, in particular in severing grinding, endeavours are made to attain the highest possible peripheral speeds of the grinding body.
In this respect, the term severing grinding is used to mean grinding through materials, preferably in bar form, with grinding wheels of relatively small thickness; that is to say, the thickness T is generally less than 0,01 of the diameter D:T.ltoreq.0.01.D.
In technical use, maximum peripheral speeds of 100 m/sec are encountered at present, but peripheral speeds of 130 m/sec and more have already been achieved in testing and laboratory work.
In this respect it will be appreciated that the stricter safety requirements which apply to high-speed grinding operations must be especially considered. Thus, fur a grinding wheel operating speed of 100 m/s, a break up (burst speed) of at least 150 m/s is required.
In accordance with the laws of mechanics, increasing the wheel speed to 1.5 times results in an increase of 2.5 times of the maximum stresses which occur in the wheel. This follows from the fact that the tangential stress at the bore of a rotating wheel does not rise linearly with the peripheral speed v, but quadratically, that is to say: EQU .sigma..sub.tang, max =prop.multidot.v.sup.2
However, the advantages of high-speed grinding are so great that this method is being used to an ever increasing extent in practice.
The most important advantages are that the cutting forces at the grain material decrease with increasing speed and the efficiency factor and the specific workpiece removal efficiency (volume of material removed) and the level of edge or profile stability increase in an overproportional manner.
The efficiency factor here is the ratio between the cross-sectional surface area severed, and the disc surface area consumed. The specific removal efficiency denotes the volume of workpiece which is removed in a unit of time per millimeter of wheel width.
Also, endeavours are made to produce severing grinding wheels which are of the smallest possible wheel thickness T. This provides the advantage that on the one hand a lower drive power is required on the part of the severing machine, and that on the other hand the degree of material waste (cutting loss=volume cut away per cut) falls.
This is important particularly in respect of expensive materials such as high-alloy steels, titanium, tungsten and the like. In addition, the degree of heating of the workpieces is lower, and this is advantageous on the one hand in regard to materials which are sensitive to heat or cracking and on the other hand in regard to protection of the environment (reducing the required content of grinding-active filler materials in the wheels).
A further requirement made on severing grinding wheels is that the wheel diameter D and thus the useful wheel surface area should be as large as possible. Such a wheel affords the possibility of cutting through larger cross-sections or longer service periods (reduction in wheel replacement costs). At the present time, severing grinding wheels of up to 1200 mm diameter can be produced by a mass production operation, and severing grinding wheels up to 1800 mm can be produced individually.
It is known however that the lateral rigidity of a wheel falls in proportion as the thickness of the wheel is decreased and the diameter of the wheel is increased, that is to say, the greater the ratio D to T of a severing grinding wheel, the greater is the tendency of the wheel to flutter and the greater is the tendency for the cut to wander off course. This can result in rough cut surfaces, cuts which are not correctly angled, and damage to the grinding wheel.
For this reason therefore, a high degree of static rigidity and a high level of dynamic stability are required, particularly for high operating speeds.
It follows from the laws of mechanics that a high degree of rigidity is linked to a high resonance frequency f.sub.res. For planar circular plates, the following formula applies: ##EQU1## in which E denotes the modulus of elasticity, .rho. denotes the density an .nu. denotes the Poisson number of the wheel.
In practicem T,D,.rho. and .nu. are virtually fixed predetermined parameters, so that an increase in stability is possible only by means of an increase in the modulus of elasticity (abbreviated to E-modulus).
However, like mechanical strength, the E-modulus is virtually predetermined with the conventional grain-resin combinations, as the selection (kind of grain, grain size, proportion of resin, etc) must be effected primarily from the points of view of the grinding operation. Now, in order to withstand the centrifugal and tangential forces which occur at the high peripheral speeds mentioned above and to increase the moduls of elasticity and thus the rigidity and stability of the wheels, with the grinding bodies which are in technical operation at the present time, it is usual to provide a fibre reinforcement. In particular, glass fibres are used in the present state of the art, as conventional organic fibres generally have insufficient heat resistance.
This latter consideration applies both to the grinding operation and also to the vuring operation, which, in the case of organic grinding body binding agents (phenol or epoxy resins and the like) is generally effected at temperatures of from 150.degree. to 190.degree. C.
In this respect, glass fibres are used:
(a) in the form of short fibres statistically distributed in the binding agent,
(b) in the form of tangled fibre fleeces,
(c) in the form of woven cloth.
In these cases, the fleeces and the woven cloth are provided with resin impregnation which generally comprises phenol resins so as to provide satisfactory transmission of force from the grinding material to the reinforcement, or so that all individual fibres are uniformly leaded (`take their share of the load`). In the course of technical development, many attempts were made to increase the effectiveness of the glass fibres in the cloth hank), special types of cloth, such as three-directional cloth, round cloth discs with preferably radial thread direction and improvements in adhesion by special surface treatment of the glass fibres (for example by silane).
In practice however it has been found that the abovementioned advances, which can be achieved by the fibre reinforcement, are nullified to a considerable extent by damage to the fibres in the production of the grinding bodies. The sharp edges of the grinding grains cut through the fibres or produce nicks in the surface of the fibres, simply in the pressing operation. The more the grinding grains are splintery and sharp-edged (for example silicon carbide, high-quality corundum), and the higher the density of the wheel structure, the more serious do such phenomena become, as a high compressing pressure is required to produce such wheels. For example, glass cloths which are removed again from uncured grinding wheels present reductions in strength of between 5 and 90%.
This damage to the fibres in the production of the grinding wheels also represents one of the main difficulties when using high strength carbon fibres as the reinforcement.
Apart from this, it should also be mentioned that, considered from the purely technical point of view, glass fibre threads, in particular the high-strength types, age relatively easily and quickly. Thus, particularly under conditions of severe dampness, the fibres may suffer from a reduction in strength of up to 30% in one hundred days. Glass fibres suffer from a further irreversible fall in strength simply in the vuring operation, the fall in strength at a curing temperature of 200.degree. C. being in fact from 10 to 20%.
In order to avoid the fall in strength in the reinforcing fibres due to damage and ageing, it has been necessary to incorporate a safety reserve in the form of higher proportions of reinforcement.
However, such an increase in the volume of reinforcement in the grinding wheel gives rise to serious disadvantages. In particular, it necessarily results in a fall in the content of wheel material which is active in the grinding operation (grain and active filler materials) and thus a drop in the efficiency factor. When using a glass fibre reinforcement, it is also found when grinding that there is a tendency for the wheel surface to be smeared by molten glass, so that the cutting capacity falls severely and the wheel grinds hotter. Moreover, similar phenomena also occur generally when using thermoplastic fibre materials.